1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method for determining the position of mobile terminals in a cellular communications system.
2. Description of Related Art
There is a widening range of cellular communications applications where it is becoming important to know the geographic position of the mobile stations (terminals) being used. For example, it is important to know the position of mobile stations being used to make or respond to emergency calls. Similarly, it is important to know the position of mobile stations being used in vehicles for fleet management purposes (e.g., taxis).
A number of methods for determining the position of cellular mobile stations either exist or have been proposed. For example, a terminal-based method for determining the position of cellular mobile stations is to manufacture the mobile stations with built-in Global Positioning System (GPS) receivers. In operation, each mobile station regularly transmits GPS-derived position information to a network on the uplink.
Another terminal-based method that has been used for determining the position of cellular mobile stations is implemented in a system marketed by TruePosition.TM.. This system has been used for determining the position of standard analog mobile radio terminals which operate in accordance with the IS-54 protocol. The TruePosition system has its own listening radio base stations and is operated independently of the cellular systems. As such, different cellular networks can share the same TruePosition position determination system. Upon request, the TruePosition system provides position information for individual mobile stations within the different cellular networks.
A network-based method for determining the position of cellular mobile stations is disclosed in commonly-assigned Swedish Patent Application No. 9603561-3 to R. Bodin. In order to determine the position of a mobile station, a handover procedure is initiated between a serving base station and the mobile station. The mobile station transmits access request signals to a new base station. The base station measures the time delay for the access request signal to travel between the mobile station and the base station. This procedure is repeated between the mobile station and one or more additional base stations. A service node in the cellular network calculates the position of the mobile station by utilizing information about the known positions of the base stations and the measured access time delays.
This network-based method of determining the position of cellular mobile stations relies on so-called "asynchronous" handovers, where the target base stations measure the access delays to the mobile station. Each access delay is used as a measure of the distance between the mobile station and the respective base station. At least two positioning handover operations are needed to obtain three such distances, which can be used in a triangulation algorithm to determine the mobile terminal's position. Notably, one distance can be obtained between the serving base station and the mobile terminal without a positioning handover. For example, in the Global System for Mobile Communications (GSM), the Timing Advance (TA) value used for time alignment of bursts can optionally be used as a representation of the distance in the serving cell. A more accurate position determination can be attained if more than two such positioning handovers are made, because more than three distances will be known. The use of more than three distance measurements compensates for some errors arising in the individual measurements.
FIG. 1 is a diagram that illustrates a handover sequence that can be used to determine the position of a mobile station, as disclosed in the above-described Swedish patent application to R. Bodin. The method 10 shown is preferably used in a GSM network. As shown, three positioning handovers are made to base stations BS1, BS2 and BSN (where N=3 in this application), respectively. Notably, a significant problem to be resolved is that these handovers are made in sequence.
For example, a first handover command (12) is transmitted from the serving base station to the mobile station whose position is to be determined. The mobile station then transmits a handover access request message (14) to a first base station BS1 whose location is known. The handover access request message is only being used by base station BS1 to measure the access delay. Consequently, base station BS1 is not required to transmit a response to the mobile station. After a predetermined time has passed, the mobile station stops waiting for a response to the access request message, and reverts back (16) to the original channel connection with the serving base station. The handover sequence is then repeated (18, 20, 22) for a second base station BS2 to measure the access delay, and again (24, 26, 28) for a third base station BS3 which again measures the respective access delay.
As mentioned above, the positioning handovers are made in sequence, with each handover (e.g., in a GSM network) taking approximately 0.5 seconds to complete. In fact, the mobile station's timeout procedure takes about 0.3 seconds alone to complete. Consequently, a shortcoming of the above-described sequential method is that the total time it takes to determine the mobile station's position is proportional to the number of cells in which the positioning handovers are made. As such, the risk of losing a call increases proportionally with the number of positioning handovers being made. Furthermore, it is difficult to compensate for timing errors in the mobile station, because these errors can vary with frequency, and the positioning handovers are made on different frequencies for the different cells. Also, the distance measurements are being performed at different points in time, which leads to inaccurate distance measurements when the mobile station continues to move. For example, if a mobile station is traveling at 200 km/h, the positioning inaccuracy caused by this movement is on the order of 0.1 km. Another shortcoming of the above-described method is that the number of measurement points is limited to the number of cells that are known to the network as being the serving cell's neighboring cells. As described below, the present invention successfully overcomes all of these shortcomings.